1. Field of the Invention
The present invention relates to a variable resistor and, more particularly, to a residue-proof (e.g., dust-proof) variable resistor equipped with a case.
2. Description of the Related Art
FIG. 21 shows a conventional variable resistor. This variable resistor 80 comprises an alumina substrate 84 having a horseshoe resistor 81 on its surface. The resistor also includes a collector electrode thin film 83 and electrode thin films 82, respectively connected to both end portions of the horseshoe resistor 81. A case 85 is provided for accommodating the alumina substrate 84 therein. Three lead terminals 86 (one of which is shown in FIG. 21) pass through the alumina substrate 84 and are respectively soldered to the electrode thin films 82 and 83. A rotor 87 is provided in the case 85 and a slider 89 is disposed on a rear surface of the rotor 87. A sealing O-ring 90 is disposed on the rotor 87 and a resin 91 is provided for sealing an opening in the rear surface of the case 85.
Furthermore, a plurality of the variable resistors 80, forming complete products, can be wrapped together to form a taping set, such that, for each resistor 80, a lead terminal 86 projecting out from the rear surface of the case 85 is pinched between a mount and an adhesive tape. Alternatively, the resistor 80 can be wrapped as a single unit, such that the lead terminal 86 is cut to a prescribed length. When mounting the variable resistor 80 in the above-described taping set onto a printed circuit board or the like, a variable resistor 80 is separated from the mount by an automatic inserter and is inserted into the printed circuit board or the like. Thereafter, the lead terminal 86 is cut and clinched and then is preliminarily fixed in place. Next, the variable resistor 80 is soldered to the printed circuit board.
However, there are a number of problems associated with the conventional variable resistor 80 described above, as enumerated below.
(1) Since the lead terminal 86 is a press-worked product, the sectional configuration thereof is rectangular. Therefore, during cutting and clinching which occurs during the mounting performed by the automatic inserter, the cutting blade initially abuts against an angular portion of the lead terminal 86. At other times, the cutting blade abuts against a planar portion of the lead terminal 86. Thus, the load applied to the cutting blade differs with the result that a cutting error is likely to occur. This, in turn, can reduce the service life of the cutting blade.
(2) Since the connection between the lead terminal 86 and the electrode thin films 82 and 83 is made by soldering, it is necessary to perform flux cleaning processing, which is a relatively expensive operation. This procedure therefore adds to the cost of the product.
(3) The use of resin 91 to perform sealing entails a complex filling operation and hardening operation, which also is a factor which increases the cost of the product.
(4) In a case where the variable resistors 80 are wrapped into a taping set, press working of the lead terminal 86 requires the use of a material having a width which is approximately twice as large as the length of the lead terminal 86. As a result, the pressing die becomes large in size which adds to the cost of the process and product produced thereby. Also, the rate (efficiency) at which the material is used becomes low. Therefore, the cost of the lead terminal increases. There also arises the inconvenience that, for example, it becomes necessary to use a different width of material between the taping set and the single unit product.